The head of Nato has warned that failure to launch military reprisals against Syria will be taken as a sign of Western “weakness”, as Barack Obama struggles to get congressional support for action.

Anders Fogh Rasmussen said he believed direct strikes against Bashar al-Assad's regime were the only way to prevent him - and potentially other rogue states - using chemical weapons again.

The comments by the secretary general came as the US president stepped up his efforts to win support from politicians on Capitol Hill.

A Senate vote on the plans for military strikes could be held as early as Wednesday, with the result still thought to be hanging in the balance. The proposals are said to be facing even tougher resistance in the House.

In an interview with the Press Association, Mr Rasmussen played down the prospect of any Nato involvement in attacks.

"Nato already plays its part as a forum for consultation among allies, but also in defence of Turkey. We have deployed Patriot missiles to ensure effective defence and protection of Turkey," he said.

"And of course we keep the situation in Syria under review."

Asked if members had no appetite for intervention in the wake of Iraq and Afghanistan, Mr Rasmussen said: "I think there is a very strong political support for action. Allies have consulted and I think there is consensus that we need a firm reply to what we have seen in Syria. The use of chemical weapons is completely unacceptable.

But he insisted: "Personally it is my firm view that it should be a military response. The aim of that should be to ensure that such attacks with chemical weapons will not happen again.

"To ensure that, we need a firm military response.

"Having said that, let me also stress that when it comes to a long-term settlement of the conflict and problems in Syria, there is no military solution.

"We need a political process leading to a political settlement."

Mr Rasmussen said he was convinced Assad would use chemical weapons again if there were no reprisals.

"A lot is at stake. The international community has a responsibility to uphold and enforce the international ban against the use of chemical weapons," he said.

"Not to act is to act. Not to act would send a very dangerous message, not only to the regime in Damascus but to dictators all over the world that they can use chemical weapons, and maybe other weapons of mass destruction, without any consequences.

Mr Rasmussen said he would not pass judgment on the democratic processes of member countries, after Prime Minister David Cameron suffered a shock defeat on the principle of military intervention in Syria.

"That is democracy. We should appreciate that we live in free and open societies with a lot of debate. And for democracies, it is not easy to make decisions on military operations," he said.

"But having said that I also think this is about the responsibility to uphold and enforce the international ban against the use of chemical weapons."

Mr Rasmussen indicated that he did not believe military strikes would draw the West into a major confrontation with Russia.

"I think what will determine the long-term relationship is our common interest in upholding international peace and stability," he said. "I think the Russians also realise that a lot is at stake."

He went on: "The consequences of no action would be serious. Non-action would send the wrong signal and there is a clear risk that it would be interpreted as weakness.

"And that way there is a clear risk of proliferation of weapons of mass destruction."